Current state-of-the-art approaches [1]}, [2]}, [3]},  for sequence tagging use the Bidirectional Long Short-Term Memory (BiLSTM) as the mapping function, and a subsequent linear-chain Conditional Random Field (CRF) as the classifier. Flair  [4]} is a variant of BiLSTM-CRF sequence tagger, which achieves the state-of-the-art performance in multiple sequence tagging datasets, including the CoNLL 2003 English and German NER datasets. In this section, we detail its components in a top-down manner.
